openvpn - separate access keys for each host behind NAT

Although there's a wealth of resources on setting up VPN servers, all seem to cover a scenario where access is granted indiscriminately to a local IP or range of IP addresses. I would like to go one step further and grant remote access to each host in the LAN individually. That is to say, user / key X connects only to ip Y, wheres user / key Z connects to IP K and so forth.